Closed-cell surfaces with enhanced drag-reduction properties

ABSTRACT

An apparatus, comprising a plurality of closed cells disposed on a surface of a substrate. Each of the closed cells has at least one dimension that is less than about 1 millimeter and are configured to hold a medium therein. The apparatus also comprises a foam that contacts the closed cells. The foam has fluid walls that include a surfactant, and bubbles of the foam layer are filled with the medium.

TECHNICAL FIELD OF THE INVENTION

The present invention is directed, in general, to a apparatus and methodfor controlling the flow resistance of a fluid on a surface.

BACKGROUND OF THE INVENTION

This section introduces aspects that may be helpful to facilitating abetter understanding of the invention. Accordingly, the statements ofthis section are to be read in this light. The statements of thissection are not to be understood as admissions about what is in theprior art or what is not in the prior art.

There is great interest in the use of engineered surfaces to reduce theflow resistance of a liquid on the surface. Some structured surfaceshaving nanometer- or micron-sized raised features have promise inapplications ranging from the transport of a liquid through a channel,to reducing the drag of a vessel traveling through a liquid. However,problems must be overcome before the full benefit of these surfaces canbe realized.

One problem is that the flow resistance of a liquid on a structuredsurface can vary dramatically with the pressure of the liquid. If thepressure of the liquid increases, then the liquid will penetrate to agreater extent into the structured surface, thereby increasing the flowresistance of the liquid on that surface. Alternatively, if the pressureof the liquid decreases, then the liquid will penetrate to a lesserextent into the structured surface, thereby decreasing the flowresistance. Flow resistance can also increase when the diffusion of airout of the liquid is sufficient to form air bubbles on the structuredsurface. For instance, the formation of air bubbles on a structuredsurface that is covering the inner surface of a pipe or channel cansignificantly increase the flow resistance of a liquid by partiallyblocking the pipe or channel cross-section.

Embodiments of the present invention overcome these deficiencies byproviding an apparatus having a structured surface that facilitatesformation of a foam that provides improved pressure stability andreduced flow resistance, as well as methods of using and manufacturingsuch an apparatus.

SUMMARY OF THE INVENTION

To address the above-discussed deficiencies, one embodiment of thepresent invention is an apparatus. The apparatus comprises a pluralityof closed cells disposed on a surface of a substrate. Each of the closedcells has at least one dimension that is less than about 1 millimeterand is configured to hold a medium therein. The apparatus also comprisesa foam contacting the closed cells. The foam has fluid walls thatinclude a surfactant, and bubbles of the foam layer are filled with themedium.

Another embodiment is a method of use that comprises controlling theflow resistance of a fluid disposed on a surface of a substrate. Themethod includes contacting a fluid with a plurality of theabove-described closed cells disposed on a surface of a substrate. Themethod further includes adjusting amounts of a surfactant and a mediumin the fluid to thereby form a foam between the fluid and the closedcells.

Yet another embodiment comprises a method of manufacture. The methodcomprises forming a plurality of the above-described closed cellsdisposed on a surface of a substrate, and contacting the closed cellswith a fluid. The method also includes introducing a surfactant and amedium into the fluid such that a foam forms between the fluid and theclosed cells.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is best understood from the following detaileddescription, when read with the accompanying figures. Various featuresmay not be drawn to scale and may be arbitrarily increased or reducedfor clarity of discussion. Reference is now made to the followingdescriptions taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 presents a cross-sectional view of an example apparatus of thepresent invention;

FIG. 2 shows a plan view of the example apparatus presented in FIG. 1;

FIGS. 3 and 4 present cross-sectional views of an example apparatus atvarious stages of a method of use according to the principles of presentinvention;

FIGS. 5 and 6 present perspective and cross-sectional views of anexample apparatus at various stages of a method of use according to theprinciples of the present invention; and

FIGS. 7-10 illustrate cross-sectional views of selected stages in anexemplary method of manufacturing an apparatus according to theprinciples of the present invention.

DETAILED DESCRIPTION

As part of the present invention, it was recognized that structuredsurfaces comprising closed cell structures have excellent stability overa range of hydrodynamic pressures, but often have high flow resistance.It was further realized that the flow resistance of closed celledstructures could be improved by forming a foam between the closed celledstructures and bulk fluid over the closed celled structures. Byseparating the flow of the over-lying fluid from the closed celledstructures, flow resistance is greatly diminished.

For the purposes of the present invention, closed cells are defined asnanostructures or microstructures having walls that enclose an open areaon all sides except for the side over which a fluid could be disposed.The term nanostructure as used herein refers to a predefined raisedfeature on a surface that has at least one dimension that is about 1micron or less. The term microstructure as used herein refers to apredefined raised feature on a surface that has at least one dimensionthat is about 1 millimeter or less.

The term medium, as used herein, refers to any gas or liquid that islocatable in the structured surface. The term fluid refers to any gas orliquid that is locatable in or on the structured surface. In some cases,e.g., the medium comprises a gas such as air or nitrogen located withinthe closed celled nano- or microstructures, and the fluid comprises aliquid such as water that is located over the closed celled nano- ormicrostructures.

One embodiment of the present invention is an apparatus. FIG. 1 presentsa cross-section view of an example apparatus 100 to illustrate certainfeatures of the present invention. FIG. 2 shows a lower magnificationplan view of the apparatus 100 along view lines 2-2 in FIG. 1. Forclarity, the medium and fluid are not depicted in FIG. 2.

As illustrated in FIGS. 1-2, the apparatus 100 comprises a plurality ofclosed cells 105 of diameter 107 are disposed on a surface 110 of asubstrate 112. In some cases, the substrate 112 is a planar substrate,and more preferably, a stack of planar substrates that are in contactwith each other. For instance, the substrate 112 can comprise aninorganic semiconductor, such as silicon or silicon-on-insulator (SOI).In other instances, however, the substrate 112 is a non-planarsubstrate, and can comprise other materials, such as plastics or metals.

Each of the closed cells 105 is a nanostructure or microstructure, e.g.,they each have at least one dimension that is less than about 1millimeter, and in some cases, less than about 1 micron. In someembodiments of the apparatus 100, such as illustrated in FIGS. 1 and 2,the one dimension of each closed cell 105 that is about 1 millimeter orless is a lateral thickness 115 of a wall 117 of the closed cell 105. Inother embodiments, the lateral thickness 115 is less than about 1micron.

For the embodiment shown in FIGS. 1-2, each closed cell 105 prescribes ahexagonal shape and shares their walls 117. Other embodiments of theclosed cell 105 can prescribe circular, square, octagonal or othergeometric shapes, and could be independent structures that do not sharewalls. It is not necessary for each of the closed cells 105 to haveshapes and dimensions that are identical to each other, although this ispreferred in some embodiments of the apparatus 100, for ease ofmanufacture and to provide a surface with uniform wetability properties.

The closed cells 105 are configured to hold medium 120 therein. E.g.,for the apparatus 100 illustrated in FIG. 1, each of the closed cells105 hold a medium 120 (e.g., a gas) therein. As also shown in FIG. 1,the apparatus 100 further comprises a foam 125 that contacts the closedcells 105. The foam 125 has fluid walls 130 that include a surfactant132, and individual bubbles 135 of the foam 125 are filled with themedium 120. For the embodiment depicted in FIG. 1, the foam layer 125 islocated between the closed cells 105 and a fluid 140 (e.g., water)located over the closed cells 105. The fluid 140 can also contain themedium 120 and surfactant 132 dissolved therein.

In some preferred embodiments of the apparatus 100, the foam 125 has astatic foam layer 145. That is, the individual bubbles 135 of staticfoam layer 145 remain substantially stationary on the closed cells 105for an extended period. E.g., in preferred embodiments, each bubble 135of the foam layer 145 is associated with a particular closed cell 105for a period, of on average, at least about 1 minute, and in some casesabout 15 minutes. Even more preferably, there is a static foam layer 145when the fluid 140 and substrate 112 are moving relative to each other(e.g., fluid 140 is moving over the substrate's surface 110, or, thesubstrate 112 is moving through the fluid 140). In other embodiments,however, the foam 125 is transient. In such cases, the bubble 135 of thefoam 125 are associated with the closed cells 105 for brief periods,e.g., less than 1 minute, and in some cases less than 1 second.Preferably in such embodiments, the foam 125 is continuously replenishedas old bubbles 135 pinch-off and enter the bulk of the fluid 140.

As also illustrated for the embodiment shown in FIG. 1, at least aportion 147 of the fluid walls 130 is substantially parallel to walls117 of the closed cells 105. That is, the fluid walls 130 projectorthogonally out from the surface 110 and thereby continue the walls 117of the closed cells 105 so that the closed cells 105 are separated fromthe fluid 140. In other embodiments, the bubbles 135 can besubstantially spherical.

In some cases, the formation of the foam 125 depends upon having themedium 120 and surfactant 132 present in the fluid 140. In particular,to form a static foam layer 145 it is important, and in some casescritical, to have certain concentrations of surfactant 132 and medium120 in the fluid 140.

It has not been previously recognized that adding a surfactant andmedium to a fluid can reduce the flow resistance experienced by a micro-or nanostructured surfaces moving with respect to the fluid.Consequently, the concentrations of medium and surfactant in the fluidare not previously recognized result-effective variables with respect toreducing flow resistance.

It is important to have enough surfactant 132 in the fluid 140 so thatthere will be sufficient numbers of surfactant molecules 150, eachcomprising polar end 152 and non-polar end 154, to form a continuouslayer of molecules 150 at the interface 155 between the fluid 140 andthe medium 120 inside of the foam 125. On the other hand, it isimportant to avoid excessive amounts of surfactant 132, because too muchof surfactant 132 can result in the formation of micelles, which couldinterfere with the stability of the fluid walls 130 separatingneighboring bubbles 135. E.g., in some preferred embodiments, theconcentration of surfactant molecules 150 in the fluid 140 ranges fromabout 0.1 to 1 wt percent.

In some cases, it is also important to have sufficient amounts of medium120 in the fluid 140 to promote foam formation. In particular, in someembodiments, it is desirable for the amount of medium 120 dissolved inthe fluid 140 to be enough to exceed a certain critical pressure of themedium 120 in the closed cell 105. When the amount of medium 120 in thefluid 140 exceeds the critical pressure, then bubbles 135 will form atthe interface 155, and merge to form the foam 125.

The critical pressure of the medium 120 in the closed cells 105 dependsupon the surface tension of the fluid 140, as well as the diameter 107of the closed cells 105. In some preferred embodiments, the pressure ofthe medium 120 in the fluid 140 is equal to or greater than the criticalpressure. This can be the case, when e.g., when the medium 120 dissolvedin the fluid 140 is in equilibrium with the medium 120 in the closedcells 105. However, in other cases, such as when the medium 120 isintroduced into the cells 105 via the openings 160, the pressure of themedium 120 dissolved in the fluid 140 can be less than the criticalpressure. The critical pressure of the medium 120 in the closed cell 105is given by: 4·γ/d, where γ equals the surface tension of the medium 120in the fluid 140 and d equals a diameter 107 of the closed cells 105.Consider as an example, embodiments where the fluid 140 is water and themedium 120 in nitrogen, and the diameter 107 of the closed cell equalsabout 10, 15 or 25 microns. Then the pressure of the medium 120 in thefluid 140 preferably exceeds the critical pressures for suchembodiments: about 216, 144 or 86 Torr, respectively.

It is undesirable, however, to have grossly excessive amount of medium120 in the fluid 140, because this will deter the formation of a staticfoam layer 145. E.g., in some embodiments, the amount of medium 120 inthe fluid 140, is no more than about 10 percent above the criticalpressure. In cases where the amount of medium 120 in the fluid 140greatly exceeds the critical pressure (e.g., more than 10 percent abovethe critical pressure), the medium 120 will continue to diffuse from thefluid 140 into the bubbles 135. This causes the bubbles 135 to growuntil the surfactant 132 can no longer stabilize the fluid walls 130,and the bubbles 135 merge together and detach from the surface, causingthe foam 125 to break down. The foam 125 will then reassemble at theinterface 155 between the fluid 140 and the medium 120, and the entireprocess repeats itself.

In some embodiments, the surface 110 of the substrate 112 has openings160 therein that couple each of the closed cells 105 to a source 170 ofthe medium 120. E.g., as illustrated in FIG. 1, the source 170 cancomprise a chamber 172 filled with the medium 120 and coupled to theopenings 160. The amount of medium 120 fed into the cells 105 form thesource can be controlled with a regulator 174 (e.g., a valve). In suchembodiments, it is desirable to adjust the pressure of medium 120 fedfrom the source 170 into the cells 105 so as to exceed the criticalpressure. In still other cases, the apparatus 100 can be configured toelectrolytically convert portions of the fluid 140 into additionalmedium 120, such as described in U.S. patent application Ser. No.11/227,735, which is incorporated by reference herein in its entirety.

As noted above in the context of FIG. 1, each surfactant molecule 150preferably has a polar end 152 and a non-polar end 154. E.g., in somepreferred embodiments, the surfactant molecules 150 comprise anionicsurfactants (e.g., sodium dodecyl sulfate), cationic surfactants (e.g.,polyethoxylated tallow amine), non-ionic surfactants (e.g., cetylalcohol), or amphoteric (zwitterionic) surfactants.

In some preferred embodiments, such as when the surfactant 132 comprisesionic surfactant molecules 150 (e.g., sodium dodecyl sulfate), the polarend 152 contacts the fluid 140 (e.g., a polar fluid such as water) andthe non-polar end 154 contacts the medium 120 (e.g., a non-polar mediumsuch as air) located inside of the foam 125. E.g., as shown in FIG. 1,the fluid walls 130 preferably each comprise two layers 180, 182 ofsurfactant molecules 150, each layer 180, 182 having polar ends 152 ofthe surfactant molecules 150 in contact with the fluid 140, and anon-polar end 154 in contact with the medium 120 located inside of thefoam 125. In other embodiments, however, different types of surfactants132 (e.g. amphoteric or nonionic surfactants) can be used. In suchcases, although there may be more complex arrangement of the surfactantmolecules 150 relative to the fluid 140 and medium 120, the surfactants132 has same desired influence on the foam stability.

The surfactant 132 plays an important role of stabilizing the fluidwalls 130, by preventing the individual bubbles 135 of the foam 125 frommerging and coalescing. It is believed that the stability conferred tofluid walls 130 is due at least in part to the repulsive forces betweensurfactant molecules 150 and attractive forces between the surfactantmolecules 150 and the medium 120 and fluid 140. As illustrated for theembodiment depicted in FIG. 1, the surfactant molecules 150self-assemble to form continuous layers 180, 182 at the interface 155between the fluid 140 and the medium 120. The polar ends 152 of thesurfactant molecules 150 in each of the two layers 180, 182 are opposedto each other and separated by a small thickness 185 (e.g., about 1micron or less) of the fluid 140. Because the polar ends 152 of thesurfactant molecules 150 repel each other, the fluid walls 130 areprevented from further thinning and are thus stabilized. As furtherillustrated, the individual bubbles 135 can have a diameter 187 that issubstantially equal to the diameter 107 of the closed cells 105.Preferably, the bubbles 135 are substantially uniform is size.

Another aspect of the invention is a method of use. E.g., embodiments ofthe apparatus of the present invention can be used in methods where itis desirable to control the flow resistance. FIGS. 3-4 and 5-6 presentcross-sectional views of example apparatuses 300, 500 at various stagesof a use that comprises controlling the flow resistance of a fluiddisposed over a surface of a substrate. The views are analogous to theview presented in FIG. 1 but at lower magnification. Any of the variousembodiments discussed above and illustrated in FIGS. 1-2 could be usedin the method. FIGS. 3-4 and 5-6 use the same reference numbers todepict analogous structures to that shown in FIGS. 1-2.

In some cases, such as discussed in the context of FIGS. 3-4, the methodis used to control flow resistance in an apparatus 300 whiletransporting a fluid through a channel. FIG. 3 presents across-sectional view of an example apparatus 300 after contacting afluid 140 with a plurality of closed cells 105 disposed on a surface 110of a substrate 112. The closed cells 105 are nano- or microstructuredclosed cells. That is, each of the closed cells 105 has at least onedimension that is less than about 1 micron or millimeter, respectively.

For the embodiment depicted in FIG. 3, the apparatus 300 is configuredas a microfluidic device, and the substrate 112 is configured tocomprise a channel 310 of the device. Contacting the fluid 140 with theclosed cells 105 may occur by putting the fluid 140 on or in theapparatus 300, e.g., in or on the channel 310. As shown in FIG. 3, theclosed cells 105 can correspond to an interior surface 110 of thechannel 310 that is configured to transport a fluid 140. The substrate112 comprises at least a portion of the structure that defines thechannel 310. The microfluidic device has at least one dimension that isabout 1 millimeter or less. For instance, one or both the width 315 andheight 320 of the channel 310 can be about 1 millimeter or less. Asillustrated in FIG. 3, the fluid 140 contacts the closed cells 105,resulting in a high flow resistance when, e.g., the fluid 140 istraveling through the channel 310.

FIG. 4 presents a cross-sectional view of the apparatus 300 afteradjusting amounts of a surfactant 132 and a medium 120 in the fluid 140to thereby form a foam 125 between the fluid 140 and the closed cells105. As a result, there is less flow resistance for the fluid 140passing through the channel 310. E.g., in some preferred embodiments,forming the foam 125 allows the fluid 140 to flow at a faster ratethrough the channel 310 for a predefined pressure head applied to thechannel 310, as compared to when the fluid 140 is in contact with theclosed cells 105 with no foam 125 there-between (e.g., such as shown inFIG. 3). In some cases, forming the foam 125 results in a slip-interface155 between the fluid 140 and the medium 120, with a consequentreduction in flow resistance. In such cases the fluid 140, at the verybottom of the fluid wall 130 adjacent to the walls 117 of the cells 105(FIG. 1), has a velocity of zero due to the non-slip interface betweenthe fluid wall 130 and the solid walls 117.

As illustrated in FIG. 4, one or more conduits 410, 415 (e.g., pipes)may be coupled to the channel 310. The conduits 410, 415 are configuredto transfer the surfactant 132 and medium 120 into the fluid 140. Incases where the medium 120 comprises a gas (e.g., air) a certain amountof the medium 120 gets dissolved in the fluid 140. At a later stage, thedissolved gas diffuses into the cells 105 and causes the bubbles 135 ofthe foam 125 to grow. In some cases predefined amounts of surfactant 132and medium 120 are introduced into the fluid 140 to achieve the desiredreduction in flow resistance, and corresponding increase in flow rate.In other cases, the amounts of surfactant 132 and medium 120 required toproduce a particular reduction in flow resistance are not known. In suchinstances, one or both of the amounts of the surfactant 132 and medium120 introduced into the fluid 140 can be incrementally adjusted whilethe flow rate of fluid 140 in the channel 310 is monitored. In somecases, the amounts of surfactant 132 and medium 120 may be reduced tocause an increase in flow resistance, and corresponding decrease in flowrate.

In other cases, such as illustrated in FIGS. 5-6, the method is used tocontrol flow resistance in an apparatus, such as a vehicle, movingthrough a fluid. That is, controlling flow resistance is done whilemoving a body through the fluid, the body having an exterior surfacecovered with the closed cells. FIG. 5 presents a perspective view ofanother example apparatus 500 after contacting a fluid 140 with aplurality of closed cells 105 disposed on a surface 110 of a substrate112. Again, closed cells 105 are nano- or microstructured closed cells.For the embodiment depicted in FIG. 5, the apparatus 500 is configuredas a vehicle, such as an on-water or underwater vehicle, and thesubstrate 112 comprises at least a portion of a body 510 of theapparatus 500. As shown in FIG. 5, the substrate surface 110 is anexternal surface of the body 510. The body 510 is configured to movethrough or on the fluid 140. In some cases, for example, the body 510comprises a hull of the vehicle. Contacting the fluid 140 with the cells105 may occur by putting the apparatus 500 on or in the fluid 140. Asdiscussed in the context of FIG. 3, when the fluid 140 contacts theclosed cells 105 without the presence of the foam 125, there results ahigh flow resistance when, e.g., the body 510 is traveling through thefluid 140.

FIG. 6 presents a cross-sectional view of the apparatus 500 (along viewline 6-6 in FIG. 5) after adjusting amounts of a surfactant 132 and amedium 120 in the fluid 140 to thereby form a foam 125 between the fluid140 and the closed cells 105. In some preferred embodiments, forming thefoam 125 allows the body 510 to travel through the fluid 140 with lessdrag for a given propulsive force applied to the body 510, as comparedto when the fluid 140 is in contact with the closed cells 105 with nofoam 125 there-between (e.g., analogous to that shown in FIG. 3). Asfurther illustrated in FIG. 6 in some cases adjusting amounts of asurfactant 132 and a medium 120 in the fluid 140 comprises introducingone or both of the surfactant 132 and medium 120 into the closed cells105 through openings 160 in the substrate's 112 surface 110. This can beadvantageous when the foam 125 comprises a transient foam, and it isdesirable to continuously replenish the surface 110 of the substrate 112with bubbles 135 (FIG. 1) of the foam 125. As discussed in the contextof FIG. 1, the medium 120 can be introduced in a controlled fashion froma medium source 170 coupled to the openings 160 via a chamber 172 andregulator 174. Preferably, the amount of medium 120 introduced into theclosed cells 105 through the openings 160 is sufficient to exceed acritical pressure of the medium 120 in the closed cells 105. Likewise,the surfactant 132 can be introduced in a controlled fashion from asurfactant source 610 and regulator 615 coupled to the openings 160. Thesurfactant 132 can be introduced via the same chamber 172 as used forintroducing the medium 120, or via a separate chamber.

Yet another embodiment of the present invention is a method ofmanufacture. FIGS. 7-10 illustrate cross-sectional views of selectedstages in an exemplary method of manufacturing an apparatus 700according to the principles of the present invention. Any of theabove-discussed embodiments of the apparatuses shown in FIG. 1-6 can bemade by the method. The same reference numbers are used to depictanalogous structures presented in FIGS. 1-6.

FIG. 7 shows the partially-completed apparatus 700 after providing asubstrate 112 and forming closed cells 105 in the substrate 112, such asdiscussed in U.S. patent application Ser. No. 11/227,663, which isincorporated by reference herein in its entirety. For instance, theclosed cells 105 can be formed on a surface 110 of the substrate 112using conventional photolithographic and wet or dry etching procedures,or by drilling into the substrate 112. E.g., forming the closed cells105 can include deep reactive ion etching the substrate 112 or otherprocedures well-known to those skilled in the art.

As further illustrated in FIG. 7, in some cases, openings 160 are formedin the surface 110 of the substrate 112. The same procedures can be usedto form the openings 160 as used to form the closed cells 105.Preferably, each closed cell 105 has at least one opening 160 formedtherein.

In some cases, as illustrated in FIG. 8, the substrate 112 is thencoupled to a second substrate 810 in which a chamber 172 has beenformed. The first and second substrates are coupled such that theopenings 160 are in communication with the chamber 172. As furtherillustrated the chamber 172 can be coupled to a source 170 of medium 120(e.g., a tank of nitrogen gas), which further includes a regulator 174configured to control the introduction of medium 120 into the cells 105.In some cases, analogous elements are present to facilitate theintroduction of surfactant.

FIG. 9 shows the partially-completed apparatus 700 after contacting theclosed cells 105 with a fluid 140, e.g., by disposing the fluid 140 onthe substrate's surface 110, or by putting the substrate 112 in thefluid 140. As illustrated in FIG. 9, in the absence of surfactant andmedium, the bulk fluid 140 will directly contact the closed cells 105.

FIG. 10 shows the partially-completed apparatus 700 after introducing asurfactant 132 and a medium 120 into the fluid 140 such that a foam 125forms between the fluid 140 and the closed cells 105. The medium 120 andsurfactant 132 can be introduced directly into the fluid 140 or via theoptional openings 160 formed in the substrate 112. In preferredembodiments, introducing the medium 120 includes dissolving an amount ofmedium 120 in the fluid 140 so as to exceed a critical pressure of themedium 120 in the closed cells 105. E.g., in some cases a partialpressure of the medium 120 (e.g., N₂ gas) in the fluid 140 (e.g., liquidH₂0) ranges from about 86 to 216 Torr and a concentration of thesurfactant 132 (e.g., sodium dodecyl sulfate) in the fluid 140 rangesfrom about 0.1 to 1 wt percent.

Although the present invention has been described in detail, those ofordinary skill in the art should understand that they could make variouschanges, substitutions and alterations herein without departing from thescope of the invention.

1-13. (canceled)
 14. A method of use, comprising, controlling the flowresistance of a fluid disposed on a surface of a substrate, including:contacting a fluid with a plurality of cells on a substrate, whereineach of said cells has at least one dimension that is less than about 1millimeter and is configured to hold a medium therein, and wherein eachof said cells has walls that laterally enclose an area of a surface ofsaid substrate and has a top opening; and adjusting amounts of asurfactant and a medium in said fluid to thereby form a foam layercontacting said walls of said cells, wherein: bubbles of said foam layerhave fluid walls that include a surfactant, said bubbles are filled withsaid medium, and said bubbles remain substantially stationary on saidtop openings of said cells for a period on average of at least about 1minute.
 15. The method of claim 14, wherein said substrate comprises achannel and said plurality of cells are located on said surface thatcomprises an inner surface of said channel, and forming said foam layerallows said fluid to flow at a faster rate through said channel for apredefined pressure head applied to said channel, as compared to whensaid fluid is in contact with said closed cells with no said foam layerthere-between.
 16. The method of claim 14, wherein said apparatuscomprises a body configured to move through said fluid and saidplurality of cells are located on said surface that comprises an outersurface of said body and wherein forming said foam layer allows saidbody to travel through said fluid with less drag for a given propulsiveforce applied to said body, as compared to when said fluid is in contactwith said cells with no said foam layer there-between.
 17. The method ofclaim 14, wherein said adjusting comprises introducing said medium intosaid fluid until a critical pressure of said medium in said cells isexceeded. 18-20. (canceled)
 21. The method of claim 14, wherein saidadjusting comprises incrementally adjusting amounts of said medium orsaid surfactant in said fluid in order to adjust said flow resistance toa target value.
 22. The method of claim 14, wherein said foam layer ispresent when at least one of said fluid and said substrate are movingrelative to each other.